Generally, permanent-magnet motors are classified into two types. One is a surface-magnet-type motor having permanent magnets adhered to an outer circumference of a rotor core and the other is an internal-magnet-type motor having permanent magnets embedded in a rotor core. For a variable-speed drive motor, the internal-magnet-type motor is appropriate.
With reference to FIG. 19, a structure of a rotor of the internal-magnet-type motor will be explained. In FIG. 19, 11 is the rotor, 12 is a rotor core, and 14 is a high-coercive-force permanent magnet. An outer circumferential area of the rotor core 12 is provided with rectangular hollows at regular pitches, the number of the hollows being equal to the number of magnetic poles. The rotor 11 illustrated in FIG. 19 has four poles, and therefore, four hollows are formed and the permanent magnets 14 are inserted therein, respectively. The permanent magnet 14 is magnetized in a radial direction of the rotor, i.e., in a direction orthogonal to a side of the rectangular section of the permanent magnet 14 that faces an air gap. The permanent magnet 14 is usually an NdFeB permanent magnet having a high coercive force so that it is not demagnetized with a load current. The rotor core 12 is formed by laminating electromagnetic sheets through which the hollows are punched. A related art of this kind is described in “Design and Control of Internal Magnet Synchronous Motor,” Takeda Yoji, et al., published by Ohm-sha. A modification of the internal type is described in Japanese Unexamined Patent Application Publication No. H07-336919. High-output motors with excellent variable speed performance are permanent-magnet-type reluctance rotating electrical machines described in Japanese Unexamined Patent Application Publication No. H11-27913 and Japanese Unexamined Patent Application Publication No. H11-136912.
A permanent-magnet-type rotating electrical machine always generates constant linkage flux from permanent magnets, to increase a voltage induced by the permanent magnets in proportion to rotation speed. When carrying out a variable-speed operation from low speed to high speed, the permanent magnets induce a very high voltage at high rotation speed. The voltage induced by the permanent magnets is applied to electronic parts of an inverter, and if the applied voltage exceeds a withstand voltage of the electronic parts, the parts will cause insulation breakage. It is necessary, therefore, to design the machine so that the flux amount of the permanent magnets is below the withstand voltage. Such a design, however, lowers the output and efficiency of the permanent-magnet-type rotating electrical machine in a low-speed zone.
If a variable-speed operation is carried out in such a way as to provide nearly a constant output from low speed to high speed, the voltage of the rotating electrical machine will reach a source voltage upper limit in a high rotation speed zone. This is because the linkage flux of the permanent magnets is constant. In the high rotation speed zone, therefore, a current necessary for providing the output will not be passed. This greatly drops the output in the high rotation speed zone and the variable-speed operation will not be carried out in a wide range up to high rotation speed. To cope with this, recent techniques of expanding a variable-speed range employ flux-weakening control described in the above-mentioned “Design and Control of Internal Magnet Synchronous Motor.” The flux-weakening control applies a demagnetizing field created with a d-axis current to the high-coercive-force permanent magnets 4, to move a magnetic operating point of the permanent magnets within a reversible range and change a flux amount. Accordingly, the internal-magnet-type rotating electrical machine performing the field-weakening control employs as an internal permanent magnet an NdFeB magnet that has a high coercive force and is not irreversibly demagnetized by the demagnetizing field.
The demagnetizing field created with a d-axis current decreases the linkage flux of the permanent magnets and the reduction in the linkage flux produces a voltage margin for the source voltage upper limit. This results in increasing a current to increase output in the high-speed zone. The voltage margin also allows rotation speed to be increased, to expand a variable speed operating range.
This technique, however, must continuously apply the demagnetizing field to the permanent magnets. For this, a d-axis current that contributes nothing to an output must always be passed, to increase an iron loss and deteriorate efficiency. In addition, the demagnetizing field produced by a d-axis current generates harmonic flux that causes a voltage increase. Such a voltage increase limits the voltage reduction achieved by the flux-weakening control. These factors make it difficult for the flux-weakening control to conduct a variable-speed operation of the internal-magnet-type rotating electrical machine at speeds over three times a base speed. In addition, the harmonic flux increases an iron loss and generates an electromagnetic force that produces vibration.
When the internal-permanent-magnet motor is applied for a drive motor of a hybrid car, the motor rotates together with an engine when only the engine is used to drive the hybrid car. In this case, a voltage induced by the permanent magnets of the motor at middle or high rotation speed exceeds a power source voltage. To cope with this, the field-weakening control must continuously pass a d-axis current. In this state, the motor only produces a loss to deteriorate an overall operating efficiency.
When the internal-permanent-magnet motor is applied for a drive motor of an electric train, the electric train sometimes carries out a coasting operation. Then, like the above-mentioned example, the flux-weakening control must continuously pass a d-axis current, so that a voltage induced by the permanent magnets will not exceed a power source voltage. In this state, the motor only produces a loss to deteriorate an overall operating efficiency.